The subscriptions page's headroom form draws §5's field and row, and .headroom leaves its markup #4404

Part one of #3834. The first try stayed red trying markup, rules and screenshots at once. The second died on a broken clone.

Done:

• [ ] The two headroom forms in src/web/pages/subscriptions.ts (lines ~225 and ~249, class="headroom row-fields") are drawn with §5's field, row and btn from src/web/ui.ts and §5's classes. Name which ones in the pull. headroom no longer appears as a class in the page's HTML.
• [ ] Each form posts the same headroom field to the same /settings/subscriptions/:id/headroom action. The number input stays about 5em wide, and at 390 px the row still wraps instead of overflowing. Today that wrapping comes from .row:has(.headroom) in content.ts, so use a §5 class that wraps.
• [ ] Do not delete rules from src/web/style/content.ts and do not edit docs/DESIGN.md.
• [ ] A test in test/subscriptions.test.ts asserts class="headroom is absent and the form and field are present. The existing tests at test/subscriptions.test.ts:606–664 and test/settings-visible.test.ts:120 still pass unchanged.
• [ ] bun x tsc --noEmit, bun scripts/design-lint.ts and bun test test/subscriptions.test.ts test/settings-visible.test.ts test/design-lint.test.ts pass.

No screenshots: the sandbox has no browser.